The present invention relates to mufflers or silencers for silencing the noise of gas inducting machinery which generate low frequency noise and have unsteady intake flow. Such machinery includes air compressors, diesel engines and othr types of gas inducting machinery.
Silencers are known for suppressing the low frequency inlet noise from such gas inducting machinery as air compressors and the like. However, these prior art mufflers generally are of the absorptive or reactive type or combinations of these. For low frequency noise, both types of mufflers generally become quite bulky.
Because of the low frequencies involved for the subject machinery of about ten to twenty Hz for the primary pulse frequency, reactive or dissipative mufflers of reasonable size, which can effectively reduce noise levels, have not been developed. One known silencer is the Atlas Copco venturi silencer, which is described in their Bulletin AHB 7676, but it is very large--with an estimated volume of about 14.5 cubic feet. It also achieves noise reduction of the fundamental frequency of only eight to ten decibels. This silencer basically is a Helmholtz resonator (as described for example in U.S. Pat. No. 4,501,341) which has been modified by the use of a venturi for the throat section of the resonator instead of the normally used straight section pipe. The use of the venturi lowers the resonant frequency of the system and increases the insertion loss of the muffler. Even though the smaller throat diameter may result in a greater silencing effect, it appears that the designers consider high throat velocities undesirable because of the resulting increased pressure drop.